1. Field of the Invention
The present invention relates to a wireless communication apparatus, a wireless communication method, and a computer program that use multiple antennas and that perform an antenna calibration process for compensating an unbalance of phase and amplitude between antenna branches. More particularly, the present invention relates to a wireless communication apparatus, a wireless communication method, and a computer program that are applied to a communication system processing broadband signals and that perform the calibration process of each antenna branch.
Specifically, the present invention relates to a wireless communication apparatus, a wireless communication method, and a computer program that are applied to a broadband communication system dividing each broadband signal into multiple subcarriers to process the broadband signal resulting from the division, as in Orthogonal Frequency Division Multiplexing (OFDM), and that perform the calibration process of each antenna branch. More particularly, the present invention relates to a wireless communication apparatus, a wireless communication method, and a computer program that are applied to a broadband communication system using multiple packet formats differing in the arrangement of each subcarrier on the frequency axis, as in Institute of Electrical and Electronics Engineers (IEEE) 802.11n, and that perform the calibration process of each antenna branch.
2. Description of the Related Art
Wireless networks draw attention as systems that are free from wiring in wired communication methods in related art. Typical standards concerning the wireless networks include IEEE 802.11 and IEEE 802.15. For example, the OFDM modulation method, which a multi-carrier method, is adopted in IEEE 802.11a/g as a standard for a wireless local area network (LAN). Although the modulation method capable of achieving a communication speed up to 54 megabits per second (Mbps) is supported in the IEEE 802.11a/g, a next-generation wireless LAN standard capable of realizing a higher bit rate is demanded.
Wireless communication technologies capable of realizing high-throughput wireless data transmission include a multi-antenna technology in which a communication apparatus includes multiple antennas. An adaptive array antenna is in widespread use as an example of the multi-antenna technology. The adaptive array antenna is a method in which the gain of each antenna element is controlled to achieve the antenna directivities appropriate for transmission and reception in order to support the communication. Specifically, in the adaptive array antenna, a signal received by each antenna element in the array antenna is weighted by using an appropriate weighting factor to control the reception directivity pattern of the entire array antenna. In addition, each transmission signal is weighted by using a weighting factor appropriate for each antenna element and the transmission signal is transmitted through the antenna element to control the transmission directivity pattern of the entire array antenna. The array antenna is realized by a method using sector antennas, in which the main robe of each antenna is directed only to a desired direction and a radio wave is not unnecessarily radiated to a direction that is not desired by suppressing the level of side robes, or by a method in which the main robe is directed to a desired mobile station and null is directed to an interference station to improve the Signal to Interference plus Noise Ratio (SINR).
Multi-Input Multi-Output (MIMO) communication draws attention as another example of the multi-antenna wireless communication technology. The MIMO is a communication method in which multiple antennas are provided in both of the transmitter and the receiver to realize spatial multiplexing streams. At the side of the transmitter, multiple transmission data items are multiplexed by space-time coding and the transmission data items are distributed between multiple transmission antennas to transmit the transmission data items through channels. In contrast, at the side of the receiver, reception signals received through multiple reception antenna through the channels are separated into multiple transmission data items by space-time decoding to obtain original data without crosstalk between the streams. With the MIMO communication method, it is possible to increase the transmission capacity in accordance with the number of antennas without expanding the frequency band to increase the communication speed. In addition, since the spatial multiplexing is used, a higher efficiency of the frequency usage is achieved. The MIMO is a communication method using channel characteristics and differs from a simple transmission-reception adaptive array. For example, IEEE 802.11n resulting from expansion of the IEEE 802.11 adopts an OFDM-MIMO communication method.
Any multi-antenna technology has a problem in that, when a radio-frequency (RF) signal passes through an RF transmission circuit or an RF reception circuit, an effect of the individual difference between active elements or parts, such as amplifiers and frequency converters, composing the circuit appears as an unbalance of phase and amplitude between the antenna branches. Particularly, the individual difference between automatic gain control (AGC) circuits in the RF reception circuit and the individual difference between power amplifiers (PAs) in the RF transmission circuit produce greater effects. The unbalance of phase and amplitude between the antenna branches can cause degradation of the antenna characteristics in the adaptive array to from a directivity that is not desired. In addition, the unbalance of phase and amplitude between the antenna branches can cause misrecognition of the channels and can inhibit acquisition of an appropriate transmission beamforming matrix in the MIMO communication, thus greatly degrading the decoding characteristics at the side of the receiver.
In order to prevent an occurrence of the unbalance of phase and amplitude between the antenna branches, it is necessary to perform calibration so that the same characteristics are achieved in the RF transmission circuit and the RF reception circuit. The calibration is generally performed in the frequency domain. A calibration factor is multiplied in the frequency domain for every subcarrier in each branch.
In a communication system processing broadband signals, it is necessary to acquire the calibration factor for every frequency band that is used. For example, the OFDM is a modulation method in which Fast Fourier Transform (FFT) is used to divide each broadband signal into multiple subcarriers and process the broadband signal resulting from the division, as described above. In this case, the antenna calibration factor is calculated for each subcarrier. Specifically, a transfer function is acquired in transmission and reception of packets including all the corresponding subcarriers to calculate the antenna calibration factor for each subcarrier corresponding to the transfer function.
For example, an array-antenna transmission apparatus is proposed, in which the entire frequency bandwidth is divided into multiple blocks and the average of the deviations in amplitude and phase between all the subcarriers in the blocks is calculated by using a fact that the deviation of the frequency response between adjacent subcarriers is small to obtain a more precise calibration value (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-348236).
If packets are transmitted and received between other terminals in the same frequency band in which the antenna calibration is performed, wrong calibration factors can possibly be acquired due to the interference of the transmission and reception of the packets between the other terminals.
In such a case, the interference can be avoided by requesting the other terminals to stop the communication for a certain time period and, then, performing the antenna calibration. For example, a wireless communication system is proposed, in which a Clear to send (CTS)-to-Self signal is transmitted to temporarily stop the transmission by the peripheral stations and a bandwidth in which calibration signals are broadcast is allocated to perform the calibration with no interference from other stations (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-33658). However, since a terminal where the packet for requesting the stop of the communication does reach can perform the communication, the interference may not fully avoided.
Practically, since a gain of only a few decibels is lost even if the antenna calibration factors that are wrongly acquired are used for the communication, it is highly likely that the communication terminal that has performed the antenna calibration does not immediately recognize the wrong calibration factors. However, since the effect of the antenna calibration factors that have been acquired continues for a few hours to one day, continuous use of the wrong calibration factors can produce a greater effect.